feedback
The part of an output signal that is looped back into the input to control or modify a system.
feedback: output signal looped back to steady the input
Feedback is a portion of a system's output signal that is deliberately routed back to its input stage. This creates a closed loop, allowing the system to compare what it is actually doing against what it is being asked to do, then correct itself. Without feedback, an amplifier or motor controller operates blindly, unaware of load changes, temperature drift, or component aging. With feedback, the system self-corrects in real time.
The two main types are negative feedback and positive feedback. Negative feedback opposes the input signal, reducing gain and distortion while improving stability and linearity. An audio amplifier uses negative feedback to lower harmonic distortion from perhaps 5 percent down to 0.1 percent. Positive feedback reinforces the input signal and is used deliberately in oscillators to sustain oscillation, but it is dangerous in amplifiers where it causes runaway and oscillation. Most industrial control systems rely on negative feedback.
Feedback in closed-loop control
Industrial process controllers depend on feedback to maintain setpoint. A temperature controller measures the actual process temperature via a thermocouple or RTD, compares it to the setpoint, and adjusts heater power to drive error toward zero. The speed of this comparison cycle, usually 10 to 100 times per second, and the strength of the correction, called the gain, determine stability. Too little gain leaves the process drifting; too much causes oscillation around setpoint.
Feedback can introduce lag and instability if the loop includes delays. A heater element takes time to respond; the sensor takes time to measure; the controller takes time to compute. If these delays are long relative to the control action, the system overshoots, oscillates, and may never settle. This is why proportional-integral-derivative (PID) tuning is critical: the three parameters are adjusted to match the loop's natural response time and lag characteristics.
The term feedback itself comes from the analogy of a person receiving information about the results of their action and using that information to adjust the next attempt. In electronics, the concept emerged in the 1920s when Harold Black invented the feedback amplifier to compensate for vacuum tube nonlinearity and drift. Today, feedback is the foundation of all automatic control, from servo motors to power supplies to process instrumentation.